How to Maneuver in a Space Suit Using the ‘Apollo Number’

Next time you’re stuck on the moon and running out of oxygen, you’d better run, not walk, back to your lunar module — especially if you’re wearing a space suit.

Scientists say those giant, bulky suits actually make running easier and walking more difficult on the moon. By combing through video and audio recordings from Apollo moonwalks, researchers have devised a mathematical method to explain how space suits affected lunar gait during the Apollo missions, and how future space suits might change the way we get around on Mars.

“Space suits are effectively reducing the gravity level by supporting part of the weight of everything that’s being transported,” said space physiology researcher Christopher Carr of the Massachusetts Institute of Technology, who co-authored the paper published Wednesday in the journal PLoS ONE. “When you’re out there, it’s like wearing a backpack with a bunch of helium balloons attached to it.”

People choose whether to walk or run at a certain speed based on what’s more efficient, Carr said. Without a space suit, this so-called “walk-run transition” can be calculated using a mathematical model based on gravity, leg length and velocity. But inside an Apollo space suit, the researchers found that the model doesn’t hold up.

“Being in a space suit is like being inside a balloon,” Carr said. “When you take a balloon and bend it, it wants to spring back into its original state.” That springiness comes from the pressurized breathing gases inside the suit, which make the garments quite stiff but also help support their weight. The bounce helps astronauts bound from leg to leg as they run, and it encourages a third kind of gait called loping, which looks like a modified version of skipping.

Without extra support from the gas-filled suit, combined with the moon’s lower gravity, astronauts could never have maneuvered in their 220-pound exoskeletons. But the self-supporting suits have disadvantages as well: While running in a suit is easier, it’s a lot harder to walk or bend over to pick things up, as shown in the video of astronaut Charles Duke below.

The moon’s low gravity already makes astronauts start running at a lower speed than they would on Earth, but space suits tip the balance even further. “We see astronauts in Apollo videos running or loping around at very low speeds,” Carr said. Using data collected from the lunar missions, the researchers calculated a new equation to describe how space suits change gait dynamics on the moon, called the Apollo Number.

The same equation could be used to come up with a “Mars Number,” which could help NASA design space suits for optimal locomotion on the red planet. Because gravity on Mars is more than twice that on the moon, Carr said the run-walk transition could be quite different.

“When we get to Mars, we don’t know exactly which gaits we’d use,” he said. “Where we would run on the moon, we might be able to walk on Mars.” But based on their calculations, the researchers say space suits will have to get a lot lighter before they could be practical on Mars.

“For sure, we need to have a lower mass suit,” Carr said, “to achieve self-support.” One possibility would be to use a nylon-spandex suit that applies pressure, he said, instead of filling the suits with a pressurized bag of gas.

Although there are improvements to be made, Carr said he has tremendous respect for the designers who created the Apollo suits. “It’s 1960s technology, but there has really been a ton of detailed design work that’s gone into the the space suits to make them comfortable,” he said. “This is literally a small spacecraft.”

Image: Eugene Cernan during Apollo 17. NASA/Harrison Schmitt.

Equation: Self-support of a space suit depends on the pressure of the suit, its cross-sectional area and tension in the suit’s materials/PLos ONE.

Video: Self-support of the Apollo space suits, as demonstrated by Charles Duke during the Apollo 16 mission. He drops a hammer on the lunar surface and must jump repeatedly to bend the knee joint of his suit and pick it up/PLoS ONE.